1. Field of the Invention
The present invention relates to PFC converters, and, more particularly, to a PFC converter including a transformer.
2. Description of the Related Art
In a switching power supply device that uses a commercial alternating-current power supply as an input power supply, supplies a steady voltage or a steady current to a load, and performs power conversion while maintaining a high power factor, a PFC converter is disposed at an input stage (see, for example, Japanese Unexamined Patent Application Publication No. 2008-537459).
FIG. 1 is a circuit diagram of a typical PFC converter in the related art. This PFC converter includes a step-up chopper circuit and a diode bridge for rectifying the voltage of a commercial alternating-current power supply AC. The step-up chopper circuit includes a capacitor C1, a choke coil L1, a switching element Q1, a diode D1, and a capacitor C2. In order to input a sinusoidal current having the same phase as an input voltage into a diode bridge DB, a switching control circuit is connected to the switching element Q1.
The step-up chopper circuit illustrated in FIG. 1 is a typical PFC converter in the related art, and is configured to output a predetermined direct-current voltage to a load with a DC-DC converter connected to the output terminal of the PFC converter. That is, the PFC converter and the DC-DC converter form a single power supply circuit.
However, in order to reduce the number of components and achieve the reduction in size and cost, the DC-DC converter sometimes also functions as a PFC converter. In this case, in order to isolate the load connected to the output terminal of the converter from the commercial power supply and step down an output voltage, an isolated transformer is used. A PFC converter including a transformer is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-161031.
However, in a case where a PFC converter including a transformer is used, the following problems arise. In a PFC converter including a transformer, the choke coil L1 illustrated in FIG. 1 is changed to the primary winding of a transformer, and the rectifier diode D1 and the smoothing capacitor C2 are connected to the secondary side of the transformer.
When a commercial alternating-current power supply is connected to such a circuit, an inrush current input into the capacitor C1 instantaneously becomes very large.
On the primary side of the transformer, a large-capacitance smoothing capacitor such as an electrolytic capacitor is not present and only the capacitor C1 that is a small-capacitance filter capacitor is present. Accordingly, at the time of connection to a commercial alternating-current power supply, an inrush current flows through the filter capacitor C1 and a superimposed voltage (L·di/dt) is generated by the inductance component of a line or a line filter (not illustrated) connected to the line and the inrush current.
Since the capacitance of the capacitor C1 is small, dt becomes very small and the superimposed voltage becomes very high.
Accordingly, in a PFC converter including a transformer, a filter capacitor and a switching element need to be high-withstand-voltage components and a superimposed voltage protection circuit is additionally needed. It is therefore difficult to reduce costs.